The present invention relates to an inspection method of piping in a nuclear power plant and, more particularly, to a method and apparatus for inspecting reactor pressure vessel drain piping inside a pedestal on the lower side of the reactor pressure vessel.
As a conventional pipe inspection method (non-destructive inspection technique) in a nuclear power plant, a method using X-ray transmission and RT (radiographic test) calculation tomography (CT) is disclosed in JP A 9-304303. Further, JP A 6-27092 discloses an ultrasonic test (hereunder, referred to as UT), using an apparatus which is absorbed on piping by utilizing suction force of a vacuum pump and moves.
When the above-mentioned conventional technique is applied for inspection of reactor pressure vessel drain piping inside a pedestal in a nuclear power plant, the following problems occur.
Several hundred control rod drive housings each fixed to the reactor pressure vessel stand inside the pedestal, and a distance therebetween is about 145 mm and narrow. Cables for operating the control rod drives are connected to lower sides of the control rod drive housings. An automatic exchanger of the control rod drives is disposed on the bottom of the pedestal, and there is little gap between the cables and the automatic exchanger. Further, a gap between the reactor pressure vessel drain piping and the ceiling of the pedestal is only about 300 mm, and an elbow portion to be inspected, which is a part of the reactor pressure vessel drain piping, is disposed at a central portion separated about 2 m from a side wall of the pedestal. Further, an ambient atmospheric dose is high, being influenced by deposits in reactor water, so that it is difficult to work inside the pedestal (particularly at the central portion) for a long time.
In this manner, the reactor pressure vessel drain piping inside the pedestal is disposed in surroundings in which it is very difficult for person to approach and directly inspect. When the conventional technique disclosed in JP A 9-304303 is applied thereto, the inspection apparatus is moved in manual, so that it is necessary for an inspector to approach to the reactor pressure vessel drain piping to be inspected and install the inspection apparatus. Therefore, it is impossible to really apply this conventional technique thereto.
Further, in the case where the conventional technique disclosed in JP A 6-27092 is applied, it is necessary to cause the inspection apparatus to be absorbed to the reactor pressure vessel drain piping, however, since the drain piping has metal heat insulator wound thereon and the heat insulator has a convex and concave shape formed on the surface thereof, sufficient absorption force can not be obtained and there is the possibility that the inspection apparatus is fallen during the inspection. That is, it is difficult to surely inspect the drain piping with high reliability.
An object of the present invention is to provide an inspection method and an inspection apparatus for piping, each of which is able to surely inspect reactor pressure vessel drain piping inside a pedestal in a nuclear power plant while reducing exposure of radiation to an inspector.
In the present invention, nuclear fuels are taken out from a nuclear reactor pressure vessel, a rail for running is mounted under reactor pressure vessel drain piping in parallel with a horizontal portion of the reactor pressure vessel drain piping inside a pedestal positioned on the lower side of the reactor pressure vessel, an inspection apparatus is mounted on the rail, and the inspection apparatus is moved along the rail to inspect the reactor pressure vessel drain piping.
Under the condition that the nuclear fuels are taken out from the inside of the reactor pressure vessel, the rail and the inspection apparatus each are mounted, whereby it is possible to reduce exposure of radiations to an inspector. Further, the reactor pressure vessel drain piping can be surely inspected by moving the inspection apparatus along the rail mounted under the drain piping in parallel to the horizontal portion of the drain piping.
Preferably, the rail is mounted at a peripheral portion inside the pedestal.
The exposure of radiations to the inspector can be further reduced by mounting the rail at the peripheral portion inside the pedestal.
Further, preferably, the inspection apparatus is provided with a mechanism for preventing the inspection apparatus from being fallen from the rail.
The reactor pressure vessel drain piping can be surely inspected by providing the mechanism for preventing the inspection apparatus from being fallen from the rail.
Further, preferably, the rail is fixed to control rod drive housings.
The rail is sufficiently fixed without providing any new fixing member.
In another invention, nuclear fuels are taken out from a nuclear reactor pressure vessel, an inspection apparatus is mounted on a beam preset under reactor pressure vessel drain piping in parallel with a horizontal portion of the reactor pressure vessel drain piping inside a pedestal positioned on the lower side of the reactor pressure vessel, and the inspection apparatus is moved along the beam to inspect the reactor pressure vessel drain piping.
Since the beam preset under the reactor pressure vessel drain piping is used as a rail for traveling the inspection apparatus, the work can be simplified in addition to the above-mentioned effects.
Another invention comprises a carriage set on a rail arranged under reactor pressure vessel drain piping in parallel with a horizontal portion of the drain piping inside a pedestal, a driving means for moving the carriage along said rail, a falling prevention mechanism for preventing the carriage from falling from the rail, a camera mounted on the carriage and monitoring a relative position between the camera and the drain piping, and a thickness measuring means mounted on the carriage and measuring the thickness of the drain piping.
Preferably, a control means for controlling the driving means, a displaying means for displaying an image monitored by the camera and an outputting means for outputting a measurement result obtained by the thickness measuring means are provided for installation outside the pedestal. Thereby, the reactor pressure vessel drain piping can be inspected by remote operation from the outside of the pedestal.